Attention-deficit/hyperactivity disorder (ADHD) is frequently diagnosed in children with Tourette syndrome (TS). The basis for this co-occurrence is uncertain. This study aimed to determine if specific pre- and perinatal risk factors, including heavy maternal smoking and severe psychosocial stress during pregnancy, were associated with one or both disorders, or neither. We compared maternal report data on pre- and perinatal risk factors on 222 children between the ages of 7 and 18 years including 45 individuals with TS alone, 52 individuals with ADHD alone, 60 individuals with condition of comorbid TS + ADHD, and 65 unaffected control children. Pre- and perinatal histories as well as psychiatric assessments were performed using standardized questionnaires and semi-structured interviews with the mothers and children. Logistic regression was used to determine the odds ratio for each variable of interest. Compared to the mothers of unaffected control children, the mothers of children with ADHD alone reported higher rates of heavy smoking (>10 cigarettes per day) during pregnancy and higher levels of severe psychosocial stress during pregnancy (OR = 13.5, p < 0.01 and OR = 6.8, p < 0.002, respectively). The TS + ADHD and the TS alone patients also had higher rates heavy maternal smoking and high levels of psychosocial stress compared to the control children, but these differences failed to reach statistical significance (heavy smoking: OR = 8.5, p < 0.052, OR = 4.6, p < 0.19, respectively; severe psychosocial stress: OR = 3.1, p < 0.07, OR = 2.6, p < 0.11, respectively). Heavy maternal smoking and severe levels psychosocial stress during pregnancy were independently associated with a diagnosis of ADHD. TS patients also had higher rates of these risk factors, but the ORs failed to reach statistical significance. Efforts are needed to reduce the frequency of these risk factors in high-risk populations. Future studies, using genetically sensitive designs, are also needed to sort out the causal pathways.